The invention relates to an osteosynthesis screw, in particular for application in translaminar vertebral-screw fixation, with a shaft, at least partially comprising a thread, and a head formed at least at one of its ends and comprising a notch arrangement for a tool.
Translaminar screw fixation on the lumbar vertebrae in the context of spinal fusion (spondylosyndesis) and operative treatment of injuries has been used for almost twenty years. The principle of translaminar screw fixation consists of the use of osteosynthesis screws to block the zygapophyseal joints, to prevent any possible movement between vertebrae; with the resulting immobilisation of the respective section of the lumbar spine safeguarding consolidation of the spondylosyndesis or healing of the injury. The screws enter on one side of the spinous process of the bone, extend through the mutual lamina, traverse the zygapophyseal joints and end up in the base of the transverse process of the lower vertebra. The free end of the screw is then located in the ventrocaudal delimitation of the transverse process, but it must not protrude at that location.
There is only limited space available for placement of translaminar screws in the dorsal structures of the vertebrae. For anatomical reasons the tolerances are very limited in regard to the most favourable screw position both from an anatomical and a mechanical perspective. Even minor deviation from the ideal position brings about a rapid decrease in effectiveness of the respective screw. For this reason, the use of a special target device is recommended for placement of the screws.
Due to the fact that in each vertebra there is only one ideal position for each of the two translaminar screws, the problem arises in the region of the spinous process of the upper vertebra, i.e. where the screws are inserted in the bone, where they cross over, in that it is not possible to position both screws ideally. Consequently, the position of the second screw must already be considered at the time of setting the first screw. Otherwise, only a very unfavourable placement option may remain for the second screw. In particular in the case of the fifth lumbar vertebra, the positioning margin is very small.
In principle the screws act like pins that block the facet joint of the spine. Because smooth pins would slide in the bone and other alternatives have not been available, normal osteosynthesis screws with a continuous thread have been used so far. This is however associated with several disadvantages: in the case of harder bones, the thread needs to be pre-cut, a procedure which is cumbersome and which increases the danger of neurotrauma or nerve damage. Due to the thread which is of no use as far as mechanical strength is concerned, the outer diameter of the screw is unnecessarily large. During the healing process which can take several months, in the region of the zygapophyseal joints, a large number of forces act upon the screws and their bedding, transversely to the screw axis. Consequently, over time, this bedding can be destroyed by the sharp ridges of the thread. Furthermore, screws provide the temptation of excessive tightening. Because the thread of translaminar screws is pre-cut so as to be continuous, i.e. the thread continues in the spinous process and the subsequent lamina, excessive tightening of the screw may cause the spinous process to break off. Conventional translaminar vertebral-screw fixation is associated with a serious disadvantage in that due to the need for positioning the screws in the spinous process so that they cross in close proximity, and due to the limited space available in this position, placement of the second screw must already be considered before placing the first screw. Otherwise it may happen that only one screw is favourably placed from an anatomical and mechanical point of view. Furthermore, the spinous process is weakened by two screw channels crossing in close proximity. This can have a negative effect if an additional loadbearing element, e.g. a bone chip, is be jammed between the spinous processes. The protruding head of conventional screws used so far, prevents any embedded arrangement. Later on, it must be possible to remove the screws again.
It is the object of the present invention to form an osteosynthesis screw of the type mentioned in the introduction, that can be screwed-in in a way that is gentle to the bone, that provides security against axial movement and against rotational movement, and that will achieve an improvement in application techniques.
According to the invention it is proposed that the shaft has a section with a thread on the end section oriented towards the head, that the remaining section of the shaft up to the free end is without a thread, that the diameter of the head corresponds at least approximately to the outer diameter of the thread and comprises an inner notch arrangement for a tool, and that the section of the shaft with thread and/or the section of the shaft without thread is noncircular, of uniform thickness e.g. trilobular or polygonal, and/or comprises elevations and/or indentations.
By having a thread only in part of the shaft and by having a large section without a thread, with a larger core diameter, damage to the bone wall can be kept very slight. Thus the thread is provided only to prevent dislocation of the osteosynthesis screw in axial direction. When in addition the section with the thread and/or the threadless section is non-circular or similar, from the very beginning after setting the screw, positive locking between the bone material and the surface of the osteosynthesis screw is achieved by elastic adaptation of the elastically deformable bone and later by regrowing bone. In this way from the very beginning and increasingly thereafter, the new screw provides security against automatic unscrewing due to repetitive load changes, a problem occurring with conventional osteosynthesis screws. Furthermore, as a result of the special configuration of the head, the osteosynthesis screw according to the invention can be embedded, i.e. it can interact along its entire length with the bone components to be blocked, without protruding. In this way potentially dangerous fracture situations can be avoided which may well occur if there was a screw head to rest against a bone component, during screwing-in of the screw.
Since security against rotation of the osteosynthesis screw does not require great forces, positive locking by elastic deformation and by regrowing bone tissue is perfectly adequate. This also provides the option of subsequent removal of the osteosynthesis screw, because applying some force to the osteosynthesis screw in rotational direction will undo this positive locking without any problems.
The measures according to the invention make possible two techniques of application. Where there is sufficient space, the screws can be implanted in the conventional way, i.e. with the head exposed on the surface of the spinous process and with the screws crossing over in the spinous process. In the second option, which in many cases is the more favourable technique, first a shorter screw can be inserted, with its head embedded. The second screw can then be implanted without any regard to the position of the first screw. This technique provides the advantage in that it does not require any compromises regarding the most favourable anatomical and mechanical screw position. Both screws can be placed optimally.
With such a osteosynthesis screw it is advantageous if the thread of the shaft is self-tapping and/or self-cutting.
A particular embodiment provides for the thread-free section of the shaft to be cylindrical or conical, with the section of the shaft comprising the thread being non-circular, of uniform thickness e.g. trilobular or polygonal, and/or comprising elevations and/or indentations. To provide security against rotation it is perfectly adequate if only the short section of the thread is e.g. trilobular. In addition, screwing-in the osteosynthesis screw and thus production of the thread in the borehole of the bone can be facilitated. Such an embodiment is advantageous in that in the deeply screwed-in section of the osteosynthesis screw, bone material can only contact a cylindrical section; in other words no positive locking can occur which would hinder subsequent undoing by turning. This is advantageous for the subsequent undoing and unscrewing of the osteosynthesis screw.
In the context of the invention it is also possible to configure both the thread and the shaft cylindrically in this region, with the threadless section of the shaft adjacent to the thread being non-circular, of uniform thickness e.g. trilobular or polygonal, and/or comprising elevations and/or indentations. Such an arrangement is e.g. be imaginable if the deeper bone material or bone tissue is not too rigid, also in respect of the regrowing material. For security against rotation, this variant is also advantageous, whereby it must always be possible to unscrew the osteosynthesis screw. Here again, the threaded section has been provided for axial securing.
Furthermore it is proposed that the section of the shaft comprising the thread be located directly adjacent to the head. For use which is gentle on the bone it is particularly advantageous if the thread does not contact the bone too deeply along the bone. Thus for a large section, merely a type of sliding-in suffices when placing the osteosynthesis screw.
To make it possible for proper blocking of the bone components to be interconnected along the entire length of the borehole, it is provided for the outer diameter of the thread-free section of the shaft to correspond at least approximately to the core diameter of the section comprising the thread. This provides even support against the bore walls along the entire length of the osteosynthesis screw, with the thread additionally engaging the bore wall only to provide security against axial sliding.
Since even a short section of an engaging thread is sufficient to provide security against axial sliding of the osteosynthesis screw, it is proposed that the axial length of the section comprising the thread be shorter than the axial length of the thread-free section of the shaft.
Since as a result of the embeddable head of the osteosynthesis screw only an inner notch arrangement for a tool can be considered, particular attention must be paid to the necessity of subsequent unscrewing of the osteosynthesis screw. For it can happen time and again that already during insertion of the tool, bone material or similar enters the inner notch arrangement for a tool. It is thus proposed that at the head, at its deepest delimitation the inner notch arrangement for a tool provide an additional recess or a pocket-like indentation. In this way, a small accommodation space is created to accommodate such material which has inadvertently entered the inner notch arrangement for a tool, so as not to impede engagement of the tool.
A further embodiment provides for the head at its free end to comprise an annular collar whose diameter is smaller than the outer diameter of the head. This advantageous construction provides better handling of such osteosynthesis screws in combined action with respective tools, and also with any protective elements or covers.
So as not to cause any weakening of the cross-section, it is proposed that the axial length of the annular section be less than half the entire length, preferably less than a third of the entire length, of the head.
An advantageous embodiment provides for the annular collar to match an annular sleeve placed onto a tool; when in place said annular sleeve encompasses the annular collar of the head during the screwing process. This makes it possible to optimally retain in the tool the osteosynthesis screw to be screwed in, in other words, exact aiming with the osteosynthesis screw is achieved. By means of such an annular sleeve, the osteosynthesis screw can also be held captive in the tool until said osteosynthesis screw has been inserted in the borehole in the bone. In addition, the annular collar is kept free of impurities and bone deposits until the tool and thus also the sleeve are withdrawn, in this way keeping the sleeve free of impurities and bone deposits.
To enable easier insertion of the osteosynthesis screw in the prepared borehole, it is proposed that the free end of the thread-free section of the shaft comprise a section which e.g. tapers off in a truncated-cone shape or in a hemispherical shape.
An additional embodiment provides for a cover for closing off the inner notch arrangement for a tool, with said cover being able to be placed onto the head in a non-positive and/or positive way. In this way the free space of the inner notch arrangement for a tool can be kept free for subsequently necessary unscrewing of the osteosynthesis screw. The regrowing bone material can thus not enter the region of the inner notch arrangement.
In this context it is advantageous if the cover comprises an annular stay pointing towards the head, said stay being able to be placed onto the annular collar of reduced diameter at the head. This ensures optimal retention of the cover, and in addition, the annular collar is kept free of regrowing bone material. After removal of such a cover, the tool can immediately be applied, together with the annular sleeve.
It is advantageous if the outer diameter of the cover at least approximately corresponds to the outer diameter of the head. This makes it easier to place the cover after placement of the osteosynthesis screw, since in practice a part fitted to the borehole, is inserted. Of course, this also facilitates any subsequent removal of the cover.
If according to a further variant of the embodiment, viewed in axial direction, the cover comprises annular stays projecting to both sides, this provides the possibility of placing such a cover onto the free end of the head of the osteosynthesis screw, with the use of a tool. With the annular stay projecting to the other side, the end of a tool can be encompassed, said tool then being used to press the cover onto the head which is already embedded in the borehole.
In conjunction with the cover which has been placed on the head of the osteosynthesis screw after inserting said screw, an optimal solution for subsequent pulling off or removal of the cover is also proposed. This solution provides for the cover, at its centre, to comprise a predetermined breaking point and/or e.g. sections connected in sector-shape by way of predetermined breaking lines, for producing an opening when inserting a tool with a coneshaped or pyramid-shaped tip. Thus it is simple to insert a tool with which the cover is penetrated. This tool can then reach behind the cover, in the centre region, like a barb. By withdrawing the tool, the cover can be removed from the head. Subsequently it only requires insertion of the tool in the osteosynthesis screw, into the inner notch arrangement for a tool, and the osteosynthesis screw can be unscrewed. After the cover has been withdrawn, not only the inner notch arrangement for a tool but also the annular collar, are readily accessible without encountering any deposits.
If the outer limit of the cover comprises one or several groove-like indentations, this also ensures captive retention of said cover in the position in place on the head of the osteosynthesis screw. The regrowing bone material will immediately also engage these groove-like indentation(s), thus preventing subsequent axial movement in relation to the bone and thus also in relation to the osteosynthesis screw.
A further embodiment is characterised by a trunnion which at least partly fills-in a section of the borehole which remains free after embedded placement of the osteosynthesis screw, said trunnion being made from an elastic, histocompatible, non-absorbable material. In this way, access to the screw head is always ensured even if the osteosynthesis screw is to be removed after having been in place for a long time. In this way the clear area of the borehole can be prevented from filling up and thus practically being closed up by bone tissue entering.
A further embodiment is characterised by a tool insertable in the inner notch arrangement for a tool, of the osteosynthesis screw, with a sleeve coaxially arranged on said tool, said sleeve accommodating in a coaxially encompassing manner at least a large part of the axial length of the head of the osteosynthesis screw. This embodiment brings about optimal alignment and retention of the osteosynthesis screw when turning said screw in. This prevents any misalignment between the screw and the tool during screwing in.